This invention relates to a system for analyzing the frequency components of an input signal. More specifically, this invention relates to a Bragg cell spectrum analyzer with laser input used for detecting and analyzing the frequencies of incoming electromagnetic signals.
Spectrum analyzers have been developed that analyze the frequency components of electromagnetic signals. Such spectrum analyzers may consist essentially of a Bragg cell, a laser beam input, an acoustic signal input and a detector array output. (See U.S. Pat. Nos. 4,328,576; 3,667,038; 3,942,109 and "Operational Integrated Optical R.F. Spectrum Analyzer," by D. Mergerian et al., Applied Optics, Vol. 19, No. 18, pp. 3023, Sept. 15, 1978).
A Bragg cell is usually a block of crystalline material approximately 1 cm.times.1 cm. in cross section and up to 10 to 20 centimeters long. A piezoelectric transducer is bonded to the end or side of the cell and tuned to the frequency band of interest. When the transducer is excited with an electrical signal, a traveling acoustic wave is set up in the cell. This causes slight changes in the refractive index of the cell material between the peaks and valleys of the acoustic pressure wave. When light is introduced at the correct angle, termed the Bragg angle, the refractions from the index changes add in phase, and Bragg diffraction takes place. A portion of the input light beam is deflected, and can be imaged onto a screen or photodetector. The amplitude of the deflected beam is proportional to the amplitude of the acoustic input and the deflection angle is proportional to the frequency of the acoustic input. If a radio frequency signal is fed into the Bragg cell, a spot of light is imaged the position of which is proportional to the signal frequency and the amplitude of which is proportional to the instantaneous signal strength. Thus, all of the modulation on the signal is preserved. If there are simultaneous multiple input signals at different frequencies within the Bragg cell bandwidth, they will be simultaneously imaged at different positions in the Bragg cell output. This is one of the key advantages of Bragg cell signal processing. Multiple signals can be processed simultaneously without the necessity for time sharing or sweeping. Another advantage is that the output is in a form suitable for further signal processing with recently developed complex detector arrays.
Nonetheless, in situations where it is desirous to use miniature semiconductor laser diodes as optical sources, for instance at 0.8 .mu.m, for the spectrum analyzers due to their small size and low power requirements, problems exist. The spatial quality and phase front uniformity of these lasers may not be adequate for the Bragg cell optics to provide diffraction limited focal spots with low side lobes for a detector array to correctly determine the frequency components of the electromagnetic signals to be analyzed. The presence of large side lobes at the focal spot limits the available dynamic range of the spectrum analyzer and may be sufficiently serious as to render the device unusable for its intended application. In addition, the failure of the laser to provide for diffraction limited focal spots may cause an overly broad major lobe and prevent the detection of two separate frequencies within the area covered by the overly broad major lobe. These problems exist because the lateral component of the laser beam is not a smooth gaussian shape, but only approximates such a shape. (see FIG. 1) The beam thus does not allow for the spatial quality and phase front uniformity that is necessary for effective spectrum analysis. The prior art suggests the use of optical lenses to provide a solution to the problem, as is shown in U.S. Pat. No. 4,253,060 which deals with the spontaneous emission problem that is prevelant with all semiconductor lasers.